The invention relates to a picture display device comprising a display tube having a display screen and an electron gun facing said screen and having a cathode centred along an electron-optical axis and a plurality of electrodes which jointly constitute a beam-forming part for generating an electron beam, said gun further comprising an electrostatic focusing lens for generating a focusing lens field, said tube further comprising deflection means for generating a deflection field which overlaps at least a part of the focusing lens field.
In the current display tubes the focusing lens of the electron gun is always placed outside the deflection field. However, on theoretical grounds it is attractive to arrange the electrostatic electron lens for focusing the electron beam on the display screen at least partly within the system of deflection coils, thus obtaining a shorter device on the one hand and a reduction of the distance between the focusing lens and the display screen on the other hand. The function of the focusing lens is to image the cathode or the cross-over of the electron beam on the display screen. The magnification occurring during imaging is proportional to the distance between the focusing lens system and the display screen and proportional to the distance between the object to be imaged and the focusing lens system, though to a lesser extent. Consequently, the shorter the first-mentioned distance, the smaller the electron-optical magnification of the spot and the better the resolution of the display device.
Particularly when the spherical aberration of the focusing lens is considered and when optimum solutions are sought for imaging the cross-over or cathode, the distance between the focusing system and the screen remains of the greatest importance. The relation stating that the extent of focusing is proportional to the inverse value of this distance remains valid. The influence of the distance between cathode and focusing lens is then of much lesser importance.
In view of the above the focusing lens should be placed as far as possible towards the display screen, i.e. as far as possible forward in the deflection field. However, the further the focusing lens is located within the system of deflection coils, the more the deflection field is counteracted by eddy currents which are generated in the electrically readily conducting components (metal cylinders, etc.) of the conventional focusing lens. Apart from the shielding action with respect to the field generated by the system of deflection coils, these eddy currents also produce magnetic fields which disturb the focusing.